1. Field of the Invention
The present invention relates to an optical element having a mirror surface configured to transmit certain part of incident light and to reflect the other part of the incident light, and also relates to an illumination apparatus and a projection display apparatus using this optical element.
2. Description of the Related Art
Heretofore, there has been known an optical element (a dichroic mirror) having a mirror surface configured to transmit certain part of incident light and to reflect the other part of the incident light. For example, such a dichroic mirror is used as a color separation element for separating color component light. Specifically, the dichroic mirror is used in a projection display apparatus in order to separate light emitted from a lamp light source into color component light beams of multiple colors.
For an apparatus in which such a dichroic mirror is employed, a target cutoff wavelength is defined which is the boundary wavelength between the wavelength band of desired transmitted light and the wavelength band of desired reflected light. Here, the cutoff wavelength of the dichroic mirror is shifted in accordance with the incident angle of light incident on the mirror surface. Specifically, as FIG. 1 shows, the cutoff wavelength is shifted to a longer wavelength side, i.e., from a position indicated with a solid line to a position indicated with a dotted line when the incident angle becomes smaller (θs−α). On the other hand, the cutoff wavelength is shifted to a shorter wavelength side, i.e., from the position indicated with the solid line to a position indicated with a dashed line when the incident angle becomes larger (θs+α). Here, a position where a cutoff wavelength reference value that separates transmission and reflection of light having a reference incident angle θs (e.g., 45°) becomes equal to the target cutoff wavelength will be referred to as a reference position.
As described above, the cutoff wavelength is shifted according to the incident angle. This causes variation in the cutoff wavelength on the entire mirror surface with respect to the target cutoff wavelength. Thus, there has been proposed a technique in which the cutoff wavelength reference value varies gradiently according to the distance from the reference position on the mirror surface. Specifically, the cutoff wavelength reference value is set to a longer wavelength side than the target cutoff wavelength for a region where the incident angle is greater than that at the reference position, i.e., for a region where the distance of the optical path of the light emitted from the lamp light source to the region is longer than that to the reference position. On the other hand, the cutoff wavelength reference value is set to a shorter wavelength side than the target cutoff wavelength for a region where the incident angle is smaller than that at the reference position, i.e., for a region where the distance of the optical path of the light emitted from the lamp light source to the region is shorter than that to the reference position.
Incidentally, a pair of fly-eye lenses having multiple microlenses (cells) has been known as an optical element for equalizing the distribution of the amounts of light beams, emitted from the lamp light source, on a light valve (a liquid crystal panel or the like). Specifically, the light emitted from each of the cells provided in the pair of fly-eye lenses is irradiated on the entire surface of the light valve.
In an optical system including the lamp light source, the pair of fly-eye lenses, and the dichroic mirror, the pair of fly-eye lenses is disposed between the lamp light source and the dichroic mirror. That is, the mirror surface of the dichroic mirror is irradiated with the light emitted from each of the cells provided in the pair of fly-eye lenses.
Generally, in the above-described dichroic mirror, the cutoff wavelength reference value varies monotonously in proportion to the distance from the reference position on the mirror surface.
The dichroic mirror in which the cutoff wavelength reference value varies monotonously can improve, to some extent, the color purity of the color component light beams (the transmitted light and the reflected light) separated by the dichroic mirror. Nonetheless, there is an expectation for further improvement in the color purity of the transmitted light and the reflected light.